1. Field of the Invention
The present invention relates generally to an image sensing apparatus and, more particularly, to an image sensing apparatus which is capable of controlling and varying the speed of response of a diaphragm in an imaging optical system.
2. Description of the Related Art
The imaging optical systems of almost all compact video cameras employ diaphragm devices of a type which utilize a so-called automatic iris arranged to automatically control exposure so that the level of an image signal is maintained at a constant value. However, in such a video camera employing the automatic iris, it is not always possible to obtain proper exposure owing to various factors such as the distribution of brightness on a subject, a background or the like. In addition, a photographer may desire to intentionally shift the exposure level to an overexposure side or an underexposure side.
For this reason, a certain type of apparatus has the aforesaid automatic iris provided with a manual-control function in order to allow the photographer to manually adjust the diaphragm as required. Such an apparatus is arranged so that the photographer can vary the aperture value by switching a switch or the like. The apparatus employs, for example, the following control methods. In the first method, during automatic control, a driving shaft for diaphragm blades is caused to rotate by a motor, while, during manual control, the driving shaft is rotated by rotating an external knob. In the second method, during automatic control, the reference value of the level of the aforesaid image signal is set to a predetermined value, while, during manual control, the aperture value can be varied in accordance with a value which is set with an external dial. In the third method, an encoder for electrically reading the angle of rotation of a shaft is provided so as to exert control so that the value of the encoder is made equal to a set value on an external dial.
In any of the conventional examples, however, in order to enhance the stability of the operation during automatic control, it is necessary to increase the time constant of diaphragm servo control. As a result, the operation during manual control lacks smoothness, thus resulting in the problem that the operation does not correspond to an actual variation in the aperture value.
In addition, if the imaging optical system is arranged to be detachably secured to the camera body, it has been impossible to completely satisfy the requirement of the ease of coupling therebetween.